GMC Envoy Code P0128 And Bad Gas Mileage

• Comments ⇓

2004 GMC Envoy.
Engine size = 4.2L
Warning lights = Check light comes and goes.
Computer codes = P0128 coolant problem.
Question: I was told at the local parts store to replace the coolant sensor and thermostat. If you agree please tell me where this parts are located.

Answer: Yes, this is most likely a bad thermostat. That is assuming that the coolant level is full. Low coolant causes air pockets and can also cause this code. The thermostat is located behind the alternator on the drivers side of the engine, as looking through the wheel well. See this link for more info on ... Replacing GMC Thermostat

Gas mileage problem with my 2002 GMC Envoy. Work done to repair this problem = replaced thermostat, replaced fuel pressure regulator, new spark plugs.
Question = I have a 2002 GMC Envoy with about 157,000 miles. (I put all the miles on it myself.) I have an issue with poor gas mileage. The vehicle used to get 18 mpg easy with the driving that I did. Now it is getting about 15 – 16 mpg and has been since January 2008. Nobody was able to tell me or figure why the vehicle was not getting as good gas mileage as it used to. Two dealerships and about 3 repair shops have seen it and the last place said – that engine is doing exactly what it is supposed to be doing. It has had spark plugs replaced at 100,000 miles and replaced again at 150,000 miles. Along with having a fuel injector service at the 150,000 miles, trying a new MAP sensor, putting on a new thermostat, new fuel pressure regulator and having the throttle body manually cleaned – nothing has helped. So it has been on engine diagnostic analyzers to no avail.

Recently, I discovered on my own, there is no vacuum going to the fuel pressure regulator from the air intake resonator box. According to the Haynes repair manual, there should be about 12 – 14 inches of vacuum on the port. I had a vacuum gage on it and drove to work – a distance of 25 miles – no vacuum was ever on the port. Hence the fuel pressure is maxed out all the time; fuel pressure is at 52 psi at idle.

To my knowledge, the fuel injectors do not leak. I have a new resonator box on order; it has not yet arrived. And I have used an inspection mirror and a flexible flashlight, I can find nothing wrong with the current resonator box; even had a copper wire all the way through the vacuum port and out the other side. Engine vacuum appears to be just fine at 19-20 inches. The engine does not burn oil or is not known to burn oil and no problems found in the EVAP system.

The throttle body plate is closed at idle and does not appear to be open. If the throttle plate was open, I could understand the loss of vacuum. Oh, before I forget it, the computer does not throw any codes at all. A compression test has never been done as far as I know.

I have noted that if I restrict the airflow by cupping my hand over the intake side of the resonator box, vacuum begins to show on the vacuum gage. Unless something is missing, it does not make since sense to have any airflow restriction just to make vacuum show up. If the new resonator box does not fix the problem, what do I do next; as the computer could be doing its magic by shifting engine operating parameters to cover something up.

Answer:
First off, there is no vacuum to the pressure regulator. Would suggest not opening and return that part. Todays vehicles are extremely sensitive to things that can cause poor fuel economy, and will almost always set a code. Without being too general, a change in tire size, high alcohol content of the fuel, incorrect type / gapped plugs, transmission shifting too late, inaccurate fuel level sensor, torque converter clutch (TCC) not locking up- or no overdrive gear, slightly rich injector operation, A/C always on, etc can all cause poor fuel economy. TCC or transmission problem would set a code. Not always turn on the SES light, but would set a code.I came up with no service bulletins for this condition.I know these things listed are pretty general to most vehicles. Would assume the dealers would have been able to find something since we know your vehicle best. Certain national markets have different alcohol contents in the fuel in the winter months to reduce emissions, and this does lower mileage a little. But most people would never notice it.

2002 GMC Envoy. Question = how to locate O2 sensor bank 1 for a 2002 Envoy.

Answer:
An 02 envoy with the 4.2 L straight 6 cylinder only has bank 1, there is no bank 2. Bank 1 sensor 1 is in the top of the exhaust manifold. The manifold is on the passenger side of the engine. Bank 1 sensor 2 is behind the catalytic converter.

Question: Sir, I have a 2004 GMC Envoy with the code po128. According to what I read on this website, the temp sensor does not exists, yet Autozone sells it and I have it. I just want to know where it is located.
 Answer: Yes, there is a temp sensor. You may be a little confused. What we say over again on this site is that the temperature sensor will not fix the problem.

ok, so your saying its thermostat or coolant level?
 Yes. Is f the coolant level is fine, then the thermostat is the problem. The code just means that the temp sensor is out of the range it should be in. If the coolant is low, the sensor cannot read the coolant temp, if the thermostat is stuck, the reading of the sensor is not what the computer expects.

ok,thanks for that. Now a different problem, we ride along and the reduce engine power light comes on and car dies. any suggestions?
 Do you have any other codes besides the P0128?

Codes were read at Autozone. Codes are p2138 and p0300.
 The P0128 could cause the reduced power, but not to likely. The p0300 is engine misfire. If there are multiple misfires, this could cause your problem. Anything from bad ignition coil(s) to worst case scenario, engine valve problem. Many times a clogged throttle body will also cause this and the stalling.

We were guessing coil but not real sure thanks so much and have a good day.

hi can you tell me where is coolant below temperature sensor 04 GMC Envoy 4.2L i try look for can not see where in motor ? Thanks. Carlos
 There is no such sensor. I am guessing that you have a check engine light with code P0128- coolant temp below thermostat regulating temperature. This code is cause by either low coolant level, or more likely- a bad thermostat.

I have a 2002 Envoy that I have to turn the key about 3 times before it will start? Seems like there is not enough fuel getting to the engine. Any suggestions?
 You should try changing the fuel filter. If it hasn't been done, or is very clogged, it will restrict the amount of fuel and cause hard starting conditions. Could also be a fuel pump problem, but you'd need to check fuel pressure to be sure. The spec is about 55psi.

I have a 2002 GMC Envoy with a bit over 100k miles, and lately the gas millage sucks. I used to be fine with the millage until i changed the fuel tank because it had a leak, after i changed that my car has been wasting so much gas. I changed the spark plugs and oxygen sensors but still the same problem, would anyone know why?
 Changing the fuel pump would not have any effect of mileage, unless possibly the gas gauge is not accurate, giving you the impression of bad economy.

I have a 2002 GMC Envoy and i have a check engine light on the code is P1870 what do i do about this?
 This code is for a slipping component inside the transmission. You are going to have to have a transmssion tech look at it. May be time for an overhaul.